The California Mechanical Code (CMC) is one of thirteen parts of the California Building Standards Code that is adopted into law every three years by the California Legislature. The other 49 U.S. states adopt similar safety codes, which generally include the following titles (preceded by ): Building Code, Electrical Code, Fire Code, Mechanical Code, Plumbing Code, Residential Code, etc.

One of the primary objectives of the mechanical code is to help ensure that heating, ventilating, and air conditioning equipment installed in buildings are designed, operated, and maintained safely. Many HVAC systems utilize natural gas, which is highly flammable and can cause explosions.
This author investigated a natural gas explosion (also called deflagration, or subsonic combustion wave) that was caused by a series of maintenance errors, a heater malfunction, and a major code violation.

The maintenance errors caused natural gas to be released into a heater room, the code violation permitted the natural gas to accumulate in the room instead of being safely vented outdoors, and the malfunction permitted the heater to re-start automatically in an unsafe state, which ignited the explosion. Two workers received serious burn injuries from the incident, but the explosion wasn’t strong enough to damage the building.

It was difficult to rank the errors and defects according to their level of egregiousness, but the worst one was undoubtedly the combined design defect and construction defect associated with the building that housed the gas-fired heater. The heater was located at ground level inside a 23-foot tall enclosure construction from concrete masonry units (i.e., cinder blocks). The architect was responsible for the defective design, which contemplated heating equipment inside the room but didn’t incorporate the code-required ventilation area. The general contractor and appliance installer were responsible for allowing the heating equipment to be installed in the room without the proper ventilation.

The CMC requires ventilation at the top of any enclosure that houses gas-fueled appliances. The purpose is to vent natural gas (which is lighter than air) in the event of a substantial release of gas into the indoor space. Allowing flammable gas to accumulate in an enclosure is the first step in the process of forming an explosive device that lacks only an ignition source to turn into a horrific fireball or a destructive blast wave. The subject room was well sealed along the upper 75% of its height but was equipped with a louvered door at the bottom that effectively allowed combustion air into the room to supply oxygen for the heating appliance. Combustion products from the heater were vented directly to the outdoors by an electric blower, and the replacement air entered through the door louvers.

When the gas pipe developed a leak (the facts weren’t entirely clear about the size of the leak was or how it began), the gas rose to the ceiling and accumulated there, displacing the air below it to the outside environment through the door louvers.

This author performed a Large Eddy Simulation (LES) of the gas accumulation phase which showed the steady-state fuel gas concentration in the upper three-quarters of the room to be substantially greater than the Upper Flammable Limit for natural gas (approximately 15 percent by volume). This fact turned out to be the sole reason the building didn’t explode – a large fraction of the fuel gas present in the room had accumulated in zones that were too rich to burn (not enough oxygen present).
Nevertheless, when the employees were instructed to enter the room and shut off the gas to the heater, their motion created a flammable zone in some portion of the lower 25% of the room’s volume. When the defective heater ignited the flammable mixture as they were exiting the room, the fireball that was created pushed flames out through the open door and burned them badly as they tried to escape. Thankfully both survived.

Posted below are two videos showing the LES simulations for Case 1 – as installed without any venting at the ceiling, and Case 2 – as required by code, with a code-compliant opening of only 150 square inches of flow area at the top of the heater room. Case 1 shows high gas concentration (red) from ceiling down to the top louver of the entry door when gas is flowing and no significant dissipation after the gas source is shut off. Case 2 shows a temporary accumulation of moderate gas concentration (green) until the gas source is shut off, after which full dissipation occurs through the upper vent. The simulation runs approximately 24x faster than real time.

This gas accumulation simulation (along with testing of the defective heater, timeline analysis of witness testimony, and plumber standard of care analyses) helped the parties reach a resolution in this case.

NFPA 86 (Ovens and Furnaces) and NFPA 87 (Fluid Heaters) recognize that some industrial heating systems are installed where electricity is not available, and heaters must be operated without the benefit of a forced-draft, clean-air purge prior to startup.

Nevertheless, natural draft furnaces can be started up safely by ensuring ventilation doors and exhaust ducts are wide open for a sufficient amount of time prior to ignition. Natural draft ventilation is driven by buoyancy forces, just like the chimney effect that occurs when exhaust from a fire rises up a chimney (i.e., because “hot air rises”). The difference with pre-ignition purge is that the buoyancy forces arise from the difference in gas density of methane and air. One thousand liters of air weighs about 1.2 kilograms, whereas one thousand liters of natural gas weighs less than 700 grams. (By comparison, helium and hydrogen are even less dense, but the density of natural gas is sufficiently low to cause a natural draft purge in a reasonable amount of time.)

The purpose of purging a furnace prior to burner light-off is to remove any combustible gases from the furnace enclosure and thereby prevent accidental ignition of an accumulation of gas from a prior unsuccessful light-off or leaking shutoff valve. When forced ventilation is used, the standards require purging the enclosure with 4 volumes of fresh air prior to light-off. In other words, if a furnace enclosure is 100 cubic meters, and the forced draft fan can be proven to deliver at least 100 cubic meters of fresh air per minute, a purge duration of 4 minutes can be programmed into the startup sequence and the code requirement will be satisfied.

However, when natural draft ventilation is the only available method of purging, determining the length of time for purge is not straightforward. Without a fan, it is more difficult to determine the exhaust gas flow rate, but more importantly, the exhaust flow rate varies with the amount of residual methane still in the furnace. As the furnace becomes more diluted with air (i.e., as the purge process dilutes the initial methane concentration down to lower values) the buoyancy driving force declines, and so does the purge rate. There is no way to ensure a certain number of “fresh air purge volumes” are forced into and out of the enclosure because the volumetric flow rate changes with time.

To overcome this problem, the Section 8.5.1.2 of NFPA 86 requires the purge time to be determined by measurement, at a time when the furnace is at normal ambient temperature. The preferred method of doing so relies on combustible gas analyzers and oxygen analyzers to continuously measure the exhaust flow leaving the furnace until the concentration falls below 25% of the LFL (lower flammability limit) of the fuel gas in air.

This author has modeled the accumulation and dissipation of natural gas in a hypothetical furnace using a large-eddy-simulation software tool called Pyrosim, which is derived from the NIST code FDS (Fire Dynamics Simulator). A video showing the process for a 20 cubic meter furnace is shown here, and a plot of exhaust concentration versus time for the simulation is also shown.

These results are not applicable to any furnace or gas source or combustion system other than the one modeled, and readers SHOULD NOT extrapolate these results to any other furnace or application. The ventilation rate depends strongly on the size of the openings (for exhaust gas outflow and fresh air inflow) and the time required to purge an actual furnace in the field could vary greatly from case to case. Furnace users are urged to consult with a purge specialist to determine the correct purge time for their own applications.

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During a recent explosion investigation, this author discovered a new failure mode that is not sufficiently addressed in NFPA’s trio of industrial heating equipment standards (NFPA 85, NFPA 86, and NFPA 87) that cover Boilers, Ovens, and Fluid Heaters, respectively. The failure mode occurs only in heating systems equipped with natural gas burners and flue gas recirculation (FGR) for control of NOx emissions. The investigation where the failure mode manifested itself happened to be concerned with a boiler explosion, but ovens, furnaces, and fluid heaters are equally capable of experiencing the same problem, if certain factors are in play.
The schematic below identifies the primary equipment that plays a role in the incident scenario. In addition to the boiler, burner, blower and natural gas source, there are two flow valves (FV-001 and FV-002) that control the amount of FGR blended with fresh air that enters the burner. On smaller boilers, FV-001 is set manually during commissioning to approximately 50% open and rarely changed, whereas FV-002 is typically an automatic valve with two discrete positions – closed (no recirculation) and normal (standard recirculation).

NFPA burner safety requirements require a pre-ignition purge at the beginning of each burner startup to help ensure the combustion chamber is free of residual fuel gas or any other combustible vapor. NFPA burner standards have included a purge requirement for at least 50 years and such requirements have reduced the rate of explosions significantly.
The goal of the purge cycle is for approximately 4 volumes of fresh air to be admitted into the combustion chamber to drive out any unwanted combustible gas or vapor. For example, if the combustion chamber has a volume (𝑉 = 100 ft3) and the blower is delivering a flow rate Vdot = 400 acfm the purge time should be 𝑡 =1.0 min. This amount of purge is almost always conservative enough to ensure combustible vapors are diluted to a nonflammable concentration in the firebox. The very first volume of fresh air purge in theory is enough to remove the combustible vapors if a plug flow model is assumed for the air flow inside the chamber. The requirement for 4 purge volumes arises from the fact that the plug flow model isn’t conservative enough if plug flow behavior is not achieved. Hence, a perfectly-stirred vessel model is used instead. The decay of fuel concentration in the firebox is exponential with time, and 4 volumes of purge air will take a 50% fuel concentration down to 1%.
However, if the purge air isn’t comprised of pure air, but rather a mixture of “flue” gas with a high concentration of unburned fuel from the prior unsuccessful burner ignition attempt, the purging process is much slower. The figure below shows the difference in decay rates between the normal case, where the purge air is 100% air, and the compromised case, where the purge air comprises 50% FGR (with residual fuel) and 50% fresh air. When purge is carried out with contaminated air, the number of purge volumes required is 8, not 4.

For the boiler explosion case described above, this author found that FV02 had been unplugged from its power source and the damper was stuck in a partially open condition. After 3 unsuccessful ignition trials in rapid succession, the spark igniter set off an internal deflagration that damaged the vessel walls such that a complete replacement of the boiler was required.